Semiconductor devices, such as integrated circuits, are manufactured by replicating patterns onto a surface of a device substrate. The replication process typically involves lithographically transferring a pattern that is on a mask onto the device substrate using an illumination source, such as electron beam, x-ray and optical.
Membrane masks are known for use in x-ray lithography. A membrane mask uses a membrane supported by a frame. The membranes are typically made of silicon, doped silicon, silicon carbide, silicon nitride, diamond, or similar material. Because x-ray wavelengths are quickly absorbed in a substrate, thin membranes are necessary so that the x-rays may be transmitted through the substrate. Typically, the membranes are less than 5 .mu.m (micrometers) thick and are thus typically delicate and expensive to manufacture. Further, x-ray absorber films used on x-ray masks absorb rather than reflect incident radiation.
Membrane masks are not used in optical lithography because conventional optical lithography uses wavelengths that are readily transmitted through thick substrates. Thus, there is no need to incur the expense and trouble of generating a delicate membrane mask for optical wavelengths.
In optical lithography, a conventional mask includes a pattern of opaque material, such as chrome, overlying a relatively thick substrate of glass or quartz, which is transparent to the wavelength of light being used. The incident light is absorbed and reflected by the opaque material and transmitted through the substrate to expose the device substrate (or an overlying photoresist layer) with the mask's pattern.
The glass or quartz substrate of conventional optical masks is free standing, i.e., without a supporting frame, and is typically several millimeters thick. Thick silicon or quartz substrates are adequately transparent for relatively long wavelengths of light, e.g., 193 or 248 nm (nanometers). However, for high resolution optical lithography shorter wavelength light, e.g., 157 nm, may be used. Thick glass or quartz substrates lack the desired transmissiveness for short wavelength light. Present efforts to develop thick transparent materials for 157 nm focus on material modification (doping) or OH removal of fused silica to increase the optical transmission.
A material that is at least partially transparent at short wavelengths is calcium fluoride (CaF.sub.2). Unfortunately, CaF.sub.2 has a high thermal expansion coefficient, approximately 40 times that of conventional glass or quartz. During production of the overlying pattern, for example using e-beam writing, a large amount of heat is typically transferred to the substrate. Thus, a substrate with a high thermal expansion coefficient will distort during production of the overlying patterns. Consequently, if the thick glass or quartz substrate in a conventional optical mask is replaced with a CaF.sub.2 substrate, e-beam writing will heat the CaF.sub.2 substrate causing the CaF.sub.2 substrate to distort resulting in distortion of the overlying pattern. This distortion may be difficult to correct for the ground rules of future device generations.
Thus, there is a need for masks that may be used for high resolution, i.e., short wavelength, optical lithography that are not distorted when the overlying pattern is generated.